Waterjet Technology – The Triangle Cut Comparison Test

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

This post is being written in Missouri, and while the old saying about “I’m from Missouri, you’re going to have to show me,” has a different origin than most folk recognize*, it is a saying that has served well over the years. We did some work once for the Navy, who were concerned that shooting high-pressure waterjets at pieces of explosive might set them off as we worked to remove the explosive from the casing. We ran tests under a wide range of conditions and said, in effect, “see it didn’t go off – it’s bound to be safe!” “No,” they replied, “we need to know what pressure causes it go off at, and then we can calculate the safety factor.” And so we built different devices that fired waterjets at pressure of up to 10 million psi, and at that pressure (and usually a fair bit below it) all the different explosives reacted. And it turned out that one of the pressures that had been tested earlier was not that far below the sensitivity pressure of one of the explosives.

That is, perhaps a little clumsily, a lead in to explain why simple answers such as “yes I can clean this,” or “yes I can cut that” often are not the best answers. One can throw a piece of steel, for example, on a cutting table and cut out a desired shape at a variety of pressures, abrasive feed rates (AFR) and cutting speeds. If the first attempt worked, then this might well be the set of cutting conditions that become part of the lore of the shop. After a while it becomes “but we’ve always done it that way,” and the fact that it could be done a lot faster with a cleaner cut, less abrasive use and at a lower cost is something that rarely gets revisited.

So how does one go about a simple set of tests to find those answers? For many years, we worked on cutting steel. Our tests were therefore designed around cutting steel samples because that gave us the most relevant information, but if your business mainly cuts aluminum, or titanium or some other material, then the test design can be modified for that reason.

The test that we use is called a “triangle” test because that is what we use. And because we did a lot of them, we bought several strips of 0.25-inch thick, 4-inch wide ASTM A108 steel so that we would have a consistent target. (Both quarter and three-eighths thick pieces have been used, depending on what was available). The dimensions aren’t that important, though the basic shape that we then cut the strips into has some advantage as I’ll explain. (It later turned out that we could have used samples only 3-inches wide, but customs die hard, and with higher pressures the original size continues to work).

Basic Triangle Shape for waterjet test cutting

Figure 1. Basic Triangle Shape for waterjet test cutting

The choice to make the sample 6-inches long is also somewhat arbitrary. We preferred to make a cutting run of about 3 minutes so that the system was relatively stable, and we had a good distance over which to make measurements, but if you have some scrap pieces that can give several triangular samples of roughly the same shape, then use those.

The sample is then placed in a holder, clamped to a strut in the cutting table and set so that the 6-inch length is uppermost and the triangle is pointing downwards.

The holder for the sample triangle

Figure 2. The holder for the sample triangle

The nozzle is placed so that it will cut from the sharp end of the triangle along the center of the 0.25-inch thickness towards the 4-inch end of the piece. The piece is set with the top of the sample at the level of the water in the cutting table. The piece is then cut – at the pressure, AFR and at a speed of 1.25 inches per minute with the cut stopped before it reaches the far end of the piece, though the test should run for at least a minute after the jet has stopped cutting all the way through the sample.

The piece is then removed from the cutting table and, for a simple comparison, the point at which the jet stopped cutting all the way through the triangle is noted.

Showing the point at which the jet stopped cutting through various samples as a function of the age of the nozzle

Figure 3. Showing the point at which the jet stopped cutting through various samples as a function of the age of the nozzle – all other cutting conditions were the same (a softer nozzle material was being tested which is why the lifetime was so short). The view of the samples is from the underside (A in Fig 1.)

An abrasive jet cuts into material in a couple of different ways – the initial smooth section where the primary contact occurs between the jet and the piece and the rougher lower section where the particles have hit and bounced once on the target, and now widen and roughen the cut. Since some work requires the quality of the first depth, we take the steel samples, and mill one side of the sample, along the lower edge of the cut until the mill reaches the depth of the cut, and then we cut off that flap of material so that the cut can be exposed. Note that the depth is measured to the top of the section where the depth varies.

Typical example of a steel triangle that has been cut and then sectioned to show the quality of the cut

Figure 4. Typical example of a steel triangle that has been cut and then sectioned to show the quality of the cut

I mentioned in an earlier article that we had compared different designs from competing manufacturers. Under exactly the same pressure, water flow and abrasive feed rates, the difference between the cutting results differed more greatly than had been expected.

Sectioned views of six samples cut by different nozzle designs, but at the same pressure, water flow, AFR and cutting speed

Figure 5. Sectioned views of six samples cut by different nozzle designs, but at the same pressure, water flow, AFR and cutting speed

There was sufficient difference that we went and bought second and third copies of different nozzles and tested them to make sure that the results were valid, and they were confirmed with those additional tests. Over the years as other manufacturers produced new designs, these were tested and added into the table – this was the result after the initial number had doubled. (The blue are results from the first nozzle series tests shown above).

Comparative depths of cut using the same pressure and AFR but twelve different commercially available nozzle designs

Figure 6. Comparative depths of cut using the same pressure and AFR but twelve different commercially available nozzle designs

There were a number of reasons for the different results, and I will explain some of those reasons as this series continues, but I will close with a simple example from one of the early comparisons that we made. We ran what is known as a factorial test. In other words the pressure was set at one of three levels and the AFR was set at one of three levels. If each test ran at one of the combination of pressures and AFR values and each combination was run once then the nine results can be shown in a table.

Depths of cut resulting from cutting at jet pressures of 30,000 to 50,000 psi and AFR of 0.6, 1.0 and 1.5 lb/min

Figure 7. Depths of cut resulting from cutting at jet pressures of 30,000 to 50,000 psi and AFR of 0.6, 1.0 and 1.5 lb/min

The results show that there is no benefit from increasing the AFR above 1 lb/minute (and later testing showed that the best AFR for that particular combination of abrasive type, and water orifice and nozzle diameters was 0.8 lb/minute).

Now most of my cutting audience will already know that value and may well be using it but remember that these tests were carried out over fifteen years ago, and at that time, the ability to save 20% or more of the abrasive cost with no loss in cutting ability was a significant result. Bear also in mind that it only took 9 tests (cutting time of around 30 minutes) to find that out.

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* The reason that the “I’m from Missouri, you’ll have to show me,” story got started was that a number of miners migrated to Colorado from Missouri. When they reached the Rockies they found that, though the ways of mining were the same, the words that were used were different. (Each mining district has its own slang). Thus they asked to be shown what the Colorado miners meant, before they could understand what the words related to.

Waterjet Cutting – Introduction to testing waterjet nozzle performance

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

In the next few posts I will be writing about some of the tests that you can run to see how a nozzle is performing. But before getting into the details of the different tests, you should recognize that this is where a little homework will be required if you are to get the most benefit from the topic.

The world that encompasses waterjet use has grown beyond the simple categories by which we used to define it. New techniques make it possible to cut materials that used to be more difficult and expensive to produce, and as practical operational pressures have increased, so the scale, precision and economics of new opportunities have developed.

It is this range of applications that makes it impractical for me to give specific advice for every situation. So instead, by explaining how to make comparisons and what some benchmarks might be, I try to allow you to better understand your system, its capabilities and both the initial performance of nozzles. Hopefully, you’ll then be able to evaluate and decide when they may best be replaced.

One lesson I learned early was that nozzles from different companies behaved in different ways and that drawing conclusions on optimal performance, for example the selection for which pressure level and nozzle size was best, using one design would not necessarily hold with a competing design. Further, there were nozzles that began their life on our system doing very well relative to others, but which quickly declined in performance. Thus, as part of an evaluation of different designs, we would test the nozzle cutting performance against a standard requirement at fixed time intervals so that we would know when it was wearing out and should be replaced.

Change in the cutting depth of a jet stream at 50,000 psi when traversed over ASTM A108 steel

Figure 1. Change in the cutting depth of a jet stream at 50,000 psi when traversed over ASTM A108 steel as a function of the time that the nozzle had been in use.

Both the shape of the curve and the effective lifetimes of different competing nozzle designs varied quite significantly. And obviously, since most folk don’t spend a lot of their time cutting through more than an inch of steel, the operational lifetimes of nozzles will vary with the requirements for the particular job. Nevertheless, the relative ages at which nozzles can no longer reach that target can differ significantly.

Comparative effective nozzle life over which, operated at a pressure of 50,000 psi, a jet could cleanly cut a path through a 1.4 inch thick steel target at a traverse rate of 1.5 inches/minute.

Figure 2. Comparative effective nozzle life over which, operated at a pressure of 50,000 psi, a jet could cleanly cut a path through a 1.4 inch thick steel target at a traverse rate of 1.5 inches/minute.

As mentioned, the tests were carried out using nozzles from several manufacturers, and at the beginning of the test, the longest lasting nozzle was not necessarily the one that produced the fastest cut, but consistently over the interval and for about twice as long as the competition, it was able to achieve the goal.

Depths of cut in steel

Figure 3. Depths of cut in steel after (top) 1,000 minutes of nozzle use, and (bottom) after 1,500 minutes of nozzle use.

In the particular case in which we made the comparison, the major interest was in achieving a clean separation of the parts, and the edge quality was not as significant a factor. In many uses of this tool that edge quality will be important and would have given a different set of numbers (as Figure 3 would indicate) than the ones that were found for our application. As a result, the judgment that the nozzle is worn out will change to a different time, and the relative ranking of the different nozzle designs may also change.

The only way in which anyone can make a rational decision on which is the best nozzle for an application and how long it will be effective is by testing the nozzle against the stated requirement. When we began the test, we anticipated that the difference between nozzles from different manufacturers when fed with water at the same flow rate and with the same quantity and quality of abrasive would not differ that much. As Figure 2 shows, we were wrong in that idea.

There are a number of different impacts that a change in nozzle design (i.e. in most cases buying a competing design over that initially used) can bring to a cutting operation. However, these impacts are also governed by the pressure at which the work is being carried out, the amount of abrasive that is used, the relative nozzle diameters (if using a conventional abrasive waterjet system) and the speed at which the cut is made. But an initial assessment of relative merit should be carried out with equivalent parameters for the different designs.

In general, however, we ran tests at a number of pressures and with varying abrasive feed rates to ensure that the comparative evaluations were fair and consistent. As a result, we found that there were a number of different factors that came into play which are not always recognized and which could bias the results that we observed.

In the posts that follow this, I will first cover some of the different tests that can be used and then go on to explain some of the results and why they sometimes make it difficult to accept a simple comparison of results when, for example, the abrasive is not the same in both cases. To give a simple example of this, consider a conventional abrasive waterjet nozzle that is operated at increasing pressure.

Increasing the pressure will improve the cutting speed and/or the cut quality, as a general rule. It will reduce the amount of abrasive that is needed but this is where the “yes, but’s . . . .” start to appear. As the pressure of the jet increases, so the amount of abrasive that is broken within the mixing chamber will also increase so that the average size of the particle coming out of the nozzle will become smaller. The amount of this size reduction is a function of the quality of the abrasive that is being used and a function of the initial size of that abrasive.

Within a certain size range, that reduction in the particle size does not significantly change the cutting performance, but if the mix contains too many small particles, particularly if the distance to the work piece is also significant, then the cutting performance can be reduced because of the particle break-up. Different nozzle designs produce different amounts of very fine material even from the same feed rate of the same abrasive into the nozzle. When the initial feed rate of the abrasive or a different abrasive is used, estimating which design and set of operating pressures is best becomes more difficult as an abstract estimation.

This is why, in the posts that follow, the comparisons are made are based on actual measurements and why I recommend that everyone test their system using more than one design/set of operating parameters so that they can be confident that the combination that they are using will provide the best combination for the job to be done.

Waterjet Technology – Fitting the water jet nozzle to the system

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

Buying a high-pressure system requires a significant amount of money, and, as a result, most folk will make a serious attempt at comparing the quality of the different systems that they are considering buying before they make that choice. Most of the expense goes into that part of the system that sits behind the nozzle and which supplies the water and (where needed) the abrasive that form the cutting/cleaning system.

Often, however, while the upstream system is the subject of such scrutiny, the nozzles themselves and the selection of abrasive often escape this level of evaluation. Both of these “parts” of the system are part of the wear cost of operations, and, as a result, the selection of the “best’ nozzle often involves operational cost considerations with less emphasis on comparative evaluations of performance. To explain this most brutally, a company may spend $250,000 on a system but then degrade the performance of that system by over 50% by choosing a nozzle system that saves the company 15% on purchase costs over that of a competitor. (I will show figures in a later post on this topic).

In the next few posts I am going to explain some of the tests that we and others have run to compare nozzle performance and some of the results that we found. I don’t intend to “name names” because the tests that I will talk about are specific to certain specific objectives, and the reason that you are running a system will likely differ from the conditions and the performance parameters that we needed to match for some specific jobs. The evaluations will range over a number of different applications and will cover some quite expensive tests as well as some very simple ones that can be run at little cost in time or money.

But, to begin, the first question relates to how you attach the nozzle to the end of the supply pipe. Here you are, if you have followed the train of thought of the last two posts on conditioning the water as it leaves the supply pipe through a long lead section or through a set of flow conditioning tubes, the water is nicely collimated and (as I will show) could under certain circumstances have a throw distance of perhaps 2,000 jet diameters or so. Yet the average jet has an effective distance of around 125 jet diameters. Why the difference? An illustrative sketch from Bruce Selberg and Clark Barker* simply makes the point.

Comparison between a typical nozzle attachment and one where the flow channel is smoothed

Figure 1. Comparison between a typical nozzle attachment and one where the flow channel is smoothed (Barker and Selberg)

Right up to the point where the small focusing nozzle is attached to the pipe on the left (a) the flow has been conditioned to give a good jet. But then, just as the flow starts to enter into the acceleration cone in the nozzle it hits the little step at the lip of the nozzle where it attaches to the pipe.

As I will mention in a later post, when a jet hits a flat surface and can’t penetrate, then it will flow out laterally along that surface. (This also happens with wind and is why places such as Chicago are referred to as “The Windy City.”) So the outer layer of the jet hits the lip, and where does it go? It runs right into the path of the central flowing jet into the nozzle and mixes right across it. So much for stable flow, that lateral disturbance turns the flow turbulent, so that it is rapidly dissipated once it gets out of the nozzle. Professors Selberg and Barker calculated the theoretical pressure of the jet coming out of the orifices and compared it with pressure values that they measured.

Measured pressure profiles plotted against the theoretical pressure (small crosses) at different distances from a typical conventional nozzle with two orifices

Figure 2. Measured pressure profiles plotted against the theoretical pressure (small crosses) at different distances from a typical conventional nozzle with two orifices.

In comparison, as a way of ensuring that the flow path into the two orifices was smooth, the two authors added a small section made of brass between the end of the pipe and the entrance to the nozzle body ((b) in Figure 1). They inserted two pins to fit into alignment holes drilled into the end of the pipe in the insert and in the nozzle body itself.

Construction of a feed section between the nozzle body and the feed pipe to stabilize the flow

Figure 3. Construction of a feed section between the nozzle body and the feed pipe to stabilize the flow (Barker and Selberg)

When the pressure profiles were taken with one of the new set of nozzles, the difference as a function of distance was quite marked.

Profiles from the nozzle design

Figure 4. Profiles from the nozzle design shown in (b) with a two-part nozzle (Barker and Selberg). Note that the standoff distance has increased for the two sets of profiles over that in Figure 2.

Further, when the depth of cut was measured after the jets were fired into blocks of Berea Sandstone at various distances from the nozzle, the improved performance was clear out to even further distances.

Depths of cut into blocks of Berea sandstone as a function of distance from the nozzle at two flow conditions

Figure 5. Depths of cut into blocks of Berea sandstone as a function of distance from the nozzle at two flow conditions (Barker and Selberg)

The addition of the flow channeling section does make the nozzle a little longer, and the cone angle of the inside of the nozzle was continued out to the diameter of the feed pipe to reduce any steps that might induce turbulence. In addition the inside of both the transition section and the nozzle were polished to a surface finish of better than 6-microinches.

The nozzles themselves were specially constructed for us using electro-formed nickel on flame-polished mandrels and were thus quite expensive. Our particular purpose, however, was in the development of a mining machine that, with the nozzles that we used, was able to peel off a slab of coal to the height of the seam and to a depth of 3 ft at a rate of advance of at least 10 ft/minute. (A later design in Germany went over 6 times as fast, when operated underground).

The advance rate was achievable because the jets were cutting a slot consistently about 2 ft ahead of the machine, and with two jets the coal between them was washed out without having to be mined. But that is a subject for a different post a t some time in the future.

Before I leave the subject, however, some folk might comment that their nozzles sit in holders that are then threaded onto the end of the pipe – thus they should be in alignment, and they are tightened until the holder is tight on the pipe. There are two caveats with this: the first is that this does not necessarily mean that the entry into the nozzle smoothly butts up against the end of the pipe, and in alignment with it. (Hence our use of pins.) In field visits, we have measured for other operators the relative distances involved and found that there can be a gap between the end of the nozzle body and the end of the pipe, both contained within the holder. Even though the two diameters are the same, the presence of the larger chamber before the entry into the nozzle will again create turbulence and a poor jet.

The fix in both cases is a small transition piece, which is simple to design and insert to fill that gap and smooth the passage though it does bring with it the second caveat. You need to make sure that the number of threads of engagement of the holder on the pipe remains enough so that the holder won’t blow off if the nozzle blocks. (One time one of ours did, but it was in a remote location, so thankfully no-one was hurt, although there was some damage as a result).

In the next post I will start to discuss the different ways that we have used, after the nozzle is in place, to make sure that the jets were doing what they were designed to and producing a jet of the quality needed.

* The information that I used in this article can be found, in more detail, in the paper: Barker, C.R. and Selberg, B.P., “Water Jet Nozzle Performance Tests”, paper A1, 4th International Symposium on Jet Cutting Technology, Canterbury, UK, April, 1978.

Waterjetting Technology – Pipe Straighteners

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

One of the advantages that became clear, even in the early days of waterjet use in mining, was that the jets cut into the rock away from the miner. It was thus a safer method of working, since it moved the person away from the zone of immediate risk. Rock has a tendency to fall when the rock under it is removed, and by using the jets to carry out the removal, so the miner is no longer as vulnerable.

But in the early days of jet use, the range of the jet was quite limited. Part of the reason for this is that the water is generally brought to the working place along the floor. It then has to be raised through bent pipes to the level of the nozzle and then turned so that the water in the pipe is flowing in the direction in which the nozzle is pointing.

Sketch of an early Russian waterjet mining monitor

Figure 1. Sketch of an early Russian waterjet mining monitor

Even though the pressure of the jet is relatively low, the volume flow rates were high and the bends leading into the nozzle set up considerable turbulence in the jet, so that the range of the jet was quite limited beyond the nozzle. There are a number of different ways of improving the range of the jet, and I will discuss these in later posts; many of these techniques apply whether the jet is being used at high volume and low pressure for mining or at higher pressures and lower flow rates for cutting into materials. But today the technique that I will discuss is the use of flow straighteners.

The two most dramatic instances that I immediately recall for their use were at the Sparwood mine in British Columbia, where the collimated jet was able to mine coal up to more than 100 ft. from the nozzle and in an underground borehole mining application where a Bureau of Mines commissioned system was able to cut a cavity to more than 30 ft. from the nozzle, which was centrally located.

Collimating jets to get better performance is not restricted to the mining industry. A visit to Disney, for example, will find jumping jets that appear to bounce from place to place (video here) (this one shows the start of the surface waves along the jet, known as Taylor instability, which grow and cause the jet to break up; and if you want to make one Zachary Carpenter has two instructional videos on how they are made (here and here).

Essentially, as those YouTube segments show, the flow straightness is achieved by dispersing the water – using a sponge – so that it flows through a large number of drinking straws. These straws act to collimate the water flow and it emerges as a glassy rod, which even acts as a light path so that light shone down it emerges at the far end. This can be used for a variety of different purposes, other than just for entertainment.

This then is the basic idea behind a flow collimator, although for larger mining flows drinking straws are too weak, and the flow volumes need to be larger. There are various designs that have been used for mining applications. Some of the earlier trials were at the Trelewis Drift mine, where the then British National Coal Board set up an experimental operation.

Sketch of Monitor used in the NCB Trials

Figure 2. Sketch of Monitor used in the NCB Trials (after Jenkins, R.W., “Hydraulic Mining” The National Coal Board Experimental Installation at Trelewis Drift Mine in the No 3 Area of the South Western Division, M.Sc. Thesis, University of Wales, 1961.)

A number of different designs were used for the flow straighteners that were located at the nozzle end of the straight pipe section leading into the nozzle:

Designs for the initial flow straighteners used at Trelewis Drift

Figure 3. Designs for the initial flow straighteners used at Trelewis Drift (after Jenkins, R.W., “Hydraulic Mining” The National Coal Board Experimental Installation at Trelewis Drift Mine in the No 3 Area of the South Western Division, M.Sc. Thesis, University of Wales, 1961.)

More recent designs, which vary according to pressure, flow rate and pipe diameter are a combination of those on the left above and those on the right. It was such a combination that allowed the Canadian miners at Sparwood to achieve production rates of 3,000 tons of coal per shift as an average over the operation of a mining section.

While the use of flow straighteners does not give any gain over having a long straight section of pipe leading into the nozzle, it can bring the flow condition up to that level in places where the geometry (or the resulting unwieldiness of the pipe) would make the long entry impractical.

One of the more interesting applications of this is in the borehole mining of minerals. Simplistically, a hole is drilled from the surface down to the seam of valuable mineral. Then a specially designed pipe is lowered through the hole with the pipe having a nozzle set on the side. Then, as the pipe rotates and is raised and lowered, the jet mines out the valuable mineral, which flows to the cavity under the pipe, where it is sucked into a jet pump and carried to the surface.

Schematic of a borehole mining operation

Figure 4. Schematic of a borehole mining operation (George Savanick)

As I mentioned at the top of the article, the jet cut a cavity some 30 ft in radius with the jet issuing through a nozzle some 0.5 inches in diameter. In order to achieve this range, it was important that the jet was properly collimated, yet the nozzle was set so that there could be no straight section.

Section showing the feed into the borehole miner nozzle

Figure 5. Section showing the feed into the borehole miner nozzle. Note the vanes in the section leading into the nozzle (George Savanick).

The turning vanes to achieve the flow collimation were designed by Lohn and Brent (4th Jet Cutting Symposium) to produce a jet equivalent to that achieved had the nozzle been attached to a straight feed.

Turning vanes used to achieve a jet capable of cutting coal to 30-ft from the nozzle

Figure 6. Turning vanes used to achieve a jet capable of cutting coal to 30-ft from the nozzle. (P.D. Lohn and D.A. Brent “Design and Test of an Inlet Nozzle Device” paper D1, 4th Int Symp on Jet Cutting Technology, Canterbury, BHRA 1978)

Tests of the performance of the nozzle showed that it produced a jet that was at least equal in performance to a nozzle with a straight feed, up to a standoff distance of 45 ft.

In simpler applications, the designs do not need to be that complicated for many simple spraying nozzles, for example, the straightener is made up of a simple piece of folded metal.

Simple flow straightener for use in low pressure and flow applications

Figure 7. Simple flow straightener for use in low pressure and flow applications.

The water has now reached the nozzle, but that is not the end of the story of the feed system, as I will start to explain, next time.